Various quality inspections are typically performed in the manufacturing process of a liquid crystal display. Among those inspections is an inspection for judging the quality of the image displayed on a display. Since it is not easy to evaluate the image quality by way of an inspection apparatus, typically, a visual inspection (i.e., sensory inspection) has been performed heretofore. In a sensory inspection, a sample image is used as a reference of the image quality being prepared, and a visual comparison is made between the sample image and the image used as the inspection object on the display to determine the quality of the image being inspected. However, since the sensory inspection depends on human visual senses, variations in judgment result, making it dependent on the inspector performing the inspection. These variations are unavoidable. It can be said that in a sensory inspection, judgment is based on information related to differences appearing on the image and areas of regions that differ in luminance. However, it is not easy to distinguish the region where local luminance differences occur from a region where a gentle gradation unevenness is present, particularly, since judgment variations depend on the inspector.
As the judgment method is replaced with the sensory inspection described above, heretofore, various methods of evaluating image quality quantitatively have been proposed as, for instance, in the gazettes of Japanese Patent Laid-Open No. Hei 10 (1998)-2800, Japanese Patent Laid-Open No. Hei 10 (1998)-96681 and Japanese Patent Laid-Open No. 2000-113188. However, it is difficult to properly extract luminance unevenness caused on the liquid crystal display from existing conventional methods. By way of example, in a conventional liquid crystal display, ideally, light emitted from a light source (called a backlight) is evenly irradiated onto a panel forming the liquid crystal display. In practice, there is to some extent a luminance gradient on the panel. Moreover, in some cases, the liquid crystal display shows a local luminance unevenness while having such an overall luminance gradient. The methods disclosed in the gazettes of Japanese Patent Laid-Open No. Hei 10 (1998)-2800 and Japanese Patent Laid-Open No. Hei 10 (1998)-96681 are aimed at a CRT display. Therefore, the proposed methods make it difficult to extract the luminance gradient displayed on the liquid crystal display, and the local luminance unevenness which is present in the luminance gradient. The method disclosed in the aforementioned referenced gazette of Japanese Patent Laid-Open No. 2000-2113188 serves its purpose for detecting unevenness on color filters for use in the liquid crystal display. Similarly, the methods disclosed in the gazettes of Japanese Patent Laid-Open No. Hei 10 (1998)-2800 and Japanese Patent Laid-Open No. Hei 10 (1998)-96681, describes the difficulties for extracting the luminance gradient displayed on the liquid crystal display and the local luminance unevenness caused therein.